Use reverse domain name for desktop file.

Needed to let flatpak export the .desktop file

https://bugzilla.gnome.org/show_bug.cgi?id=786783

configure.ac

@@ -583,7 +583,7 @@ AC_SUBST([ENABLE_CODING_STYLE_CHECKS])
 AC_CONFIG_FILES([
    Makefile
    data/Makefile
-   data/empathy.desktop.in
+   data/org.gnome.Empathy.desktop.in
    data/icons/Makefile
    data/themes/Makefile
    extensions/Makefile

data/Makefile.am

@@ -2,7 +2,7 @@ SUBDIRS = icons themes
 
 desktopdir = $(datadir)/applications
 desktop_in_files = \
-	empathy.desktop.in \
+	org.gnome.Empathy.desktop.in \
 	$(NULL)
 desktop_DATA = $(desktop_in_files:.desktop.in=.desktop)
 @INTLTOOL_DESKTOP_RULE@